Cell aggregates appeared in cultures of human bone marrow-derived UE7T-13 mesechymal stem cells, which had been serially cultured in a growth factor defined, serum-free medium (D-hESF10) reported here, followed by culture in an osteoblastic differentiation medium, for 20 days. Staining of the aggregates with Alizarin Red S, which demonstrates the presence of calcium, indicates that these cells retain their capacity to differentiate into osteoblasts. For further details, see paper by Mimura et al. pp. 181-187.